This invention relates to solenoid operated fluid control devices, and more particularly to a variable force solenoid valve assembly adapted for use in an automatic transmission controller for controllably reducing a fluid inlet pressure to an outlet control pressure.
In the past, automatic transmissions used in motor vehicles were typically controlled through fluidic control systems incorporating numerous piston and cylinder assemblies. Although such devices operate satisfactorily, they have several drawbacks including high cost attributable to the high precision necessary in machining the components and the necessity of providing and assembling numerous components. Further, such systems are restricted in their operational capabilities. Modern motor vehicles are incorporting increasing numbers of electronically controlled sub-systems and particular attention is presently being directed toward designing electronically controlled automatic transmission controllers. This invention relates specifically to an improved variable force solenoid valve assembly which can be employed in such a controller device.
Variable force solenoid valve assemblies are used in electronic transmission controllers to provide control over fluid pressures which are to be provided in response to an electrical input signal supplied to the solenoid. In particular, variable force solenoid valve assemblies are employed to provide regulation of the "shift-feel" of an automatic transmission. "Shift-feel" is the sudden and harsh impact felt by the vehicle operator due to engagement and disengagement of the transmission gearing upon energiziation of ON/OFF solenoid valves, typically provided in such electronic controllers. Such sudden and harsh shift-feel is extremely undesireable. Variable force solenoid valve assemblies permit the calibration and fine adjustment of shift-feel to provide a smoother transition during transmission gear changes.
A number of significant design challenges are presented in designing a variable force solenoid valve assembly for controllably regulating the fluid pressure delivered to an outlet port of a transmission controller relative to the inlet line pressure. Controlled energization of the solenoid enables the control pressure to be effectively modulated. However, precise regulation of the control pressure necessitates a highly stable fluid flow condition which is directly influenced by the design of the flow control valving components and the fluid communication therethrough. Stable flow conditions tend to minimize undesireable self-excited oscillation of the valving components thereby promoting hydraulic equilibrium conditions.
Typically, variable force solenoid devices are provided with fluid control components consisting of a spool valve having a stem and two radially extending lobes or lands. The spool valve is movably confined within a valve body to create flow restrictions for pressure regulation. Fluid passages are provided in the valve body which communicate with the various surfaces of the spool valve. Commonly, fluid at inlet pressure is delivered to a flow restriction provided between the two spool valve lands. The flow restriction enables the fluid pressure to be reduced to a desired outlet control pressure. However, the fluid forces acting on the two lands of the spool valve generate a negative hydraulic damping characteristic. Negative damping produces self-excited vibration or oscillation of the spool valve during fluid flow conditions. Such self-excited oscillation inhibits accurate regulation of the control pressure. Additionally, the self-excited oscillation of the spool valve makes the variable force solenoid valve unstable and difficult to calibrate.